The present invention relates to pixel processors for converting high level picture descriptions to bit maps controlling pixel based displays, and in particular to a pixel processor providing bit maps for half tone displays.
A typical laser, ink jet or impact type dot matrix printer organizes a page to be printed into an array of dots, or pixels, and selectively inks ("turns on") various pixels of the array to produce a printed page. An internal or external computer controls the printer in response to pixel data words of the bit map of a page to be printed. Each bit of each pixel data word corresponds to a pixel and indicates whether the printer is to turn on the corresponding pixel.
An object-oriented graphic design system describes, manipulates and stores a graphic design using a high level picture description, a set of data describing various graphic objects forming the design. For example, a picture description may describe a rectangle by indicating coordinates of its corners, the thickness of its edges, its fill pattern and other attributes. A computer internal or external to the printer converts the picture description into a bit map when a user requests a print out of the graphic design.
Dot matrix printers generate a graphic page by selectively inking or not inking various pixels, and all pixels are inked with the same intensity. However, half toning techniques are often employed to print a graphic design including graphic objects having varying levels of apparent color or monochrome intensity. In a half toning system, a page to be printed is mapped onto an array of half tone cells, each half tone cell itself being a small rectangular array of pixels. Each pixel of a half tone cell is assigned a different intensity threshold level, for example, from 1 to 100, and each graphic object in a design is assigned an intensity level also on a scale of 1 to 100. The picture description indicates the intensity level of each graphic object. When the pixel processor processes the picture description to create a bit map, it compares the desired intensity level of the object to the intensity threshold level of each pixel included in the object. If the intensity threshold level is less than the intensity level of the object, the pixel processor sets the bit corresponding to the pixel so that the pixel forms a part of the object in the print out. Otherwise the pixel processor does not set the bit. For an object of intensity 100, all pixels covered by the object are turned on because all pixels have assigned intensity threshold levels of 100 or less. For an object of intensity 50, perhaps only half of the pixels covered by the object turn on because these pixels have assigned intensity levels greater than 50. Since pixels are small and closely spaced the gaps in an object caused by the missing pixels are not readily apparent to an observer. However, since fewer pixels form an object of intensity 50 than an object of intensity 100, the intensity 50 object appears fainter than the intensity 100 object.
Half toning can add depth, shading and other interesting effects to a graphic print, but the additional processing steps required for a computer to implement a half tone scheme substantially increases the time required by the computer to convert a picture description into a bit map.